1. Field of the Invention
The present invention relates to a multiple material golf club head. More specifically, the present invention relates to a multiple material golf club head with a compression-molded, thin-walled aft body.
2. Description of the Related Art
There are various problems with the current process for manufacturing multiple material golf club heads. For example, in a standard compression molding process, the hard metal tooling on both sides of the molding part makes it impossible to create undercuts without significantly increasing tool complexity. Another problem lies in the fact that standard molding compounds are not designed to be used in parts with very thin walls. When wall thicknesses are less than approximately 0.080 inches, it is difficult to compression mold most standard molding compounds. Furthermore, standard molding compounds are not as strong, stiff, or tough as laminated composites made with similar matrix and fiber types.
Laminates are typically made up of layers of aligned fibers embedded in a matrix. Each layer, or ply, has a minimum thickness that is predetermined by the raw materials when they are purchased. Plies in a manufactured part can be made thicker by stacking two or more layers of the same fiber orientation on top of one another, but there is no reasonable way to create thinner plies without purchasing different, more expensive materials. The limitation on the thickness of plies creates design constraints and limits the efficiency of even the best designs. For example, if a quasi-isotropic symmetric laminate is desired, there must be at least six plies used in order to create a [0, 60, −60]s laminate. A more common approach is to use eight plies and a [0, 45, −45, 90]s laminate. If, for example, the plies are 0.005 inches thick and eight plies must be used, the minimum part thickness is 0.040 inches. Even if analysis shows that 0.040 inches is thicker than necessary for the structural requirements of the part, the designer is limited by this minimum thickness. This leads to inefficient parts that are overbuilt and heavier than they need to be. Laminate composites also are not ideal because the raw materials typically used to make laminates are expensive. This cost is compounded by the very high scrap rate involved in molding them. Furthermore, the use of prepreg material requires hand placement of each layer of material into a mold, a time-consuming and labor-intensive process.
Another problem lies in the fact that latex bladders, which allow manufacturers to avoid undercut constraints, cause parts to lose definition on their inside surfaces. Metal tooling dictates the outer molding line (OML) of the parts quite well, but the part thickness and inner molding line (IML) of the molded parts are determined by the number of plies placed in each area and the amount of pressure exerted on the area by the bladder during the cure. As a result, it is difficult to predict the mass properties of a multiple-material body before a part is made.
One-piece bladder molded driver bodies also do not work well with a body-over-face joint. Bladder molded multiple material driver design had been restricted to body-under-face joints so that the body bond surface is a well controlled OML surface. The lack of precision on the inside of the head, however, makes it difficult to control the geometry of the body where it would meet up with the face.
Another problem lies with the fact that typical epoxy-based prepregs take at least twenty to thirty minutes, and often longer, to cure. In one multiple material golf club head fabrication process, the latex bladders used to apply pressure during the cure cycle can only be used two or three times before they need to be discarded. As such, bladders are a significant cost in the current multiple material golf club manufacturing process.